High density interconnect apparatus

ABSTRACT

The invention comprises a plurality of stacked planar processing circuit boards surrounded on at least one side by a plurality of memory boards located substantially perpendicular to the planar processing boards, the processing and memory boards connected by orthogonal interconnect modules. The orthogonal interconnect modules allow closely-spaced orthogonal connection of the processing boards to the memory boards. The memory boards are of a densely packed design having a plurality of removeable memory chip stacks located on the memory boards.

This is a division of application Ser. No. 07/618,603, filed Nov. 27,1990, now U.S. Pat. No. 5,167,511.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a high density interconnect apparatusfor connecting a plurality of central processing boards with a pluralityof memory boards in a close configuration. More particularly, theinvention comprises a plurality of stacked central processing boardssurrounded on two sides by a plurality of memory boards located inplanes perpendicular to the central processing board planes andconnected by orthogonal interconnect modules.

BACKGROUND OF THE INVENTION

Large electrical devices such as supercomputers of the type manufacturedby Cray Research, Inc., the assignee of the present invention, areconstructed of a large plurality of integrated circuit chips for bothprocessing and memory. In order to increase the processing speed ofthese electrical devices, the processors are being connected closer toone another to increase the speed of the units.

In the prior art, the connections between the central processing boardsand memory boards of multiprocessor systems have been both cumbersomeand long. As a result, the processing speed of the systems has beenlimited by the speed with which electronic signals can physically travelalong connections, which is approximately one nanosecond per foot oflength traveled. In addition, such systems have been difficult todisassemble for repair.

Further adding to the physical length of connections between theprocessing units and memory is the concept of a "completely connectedsystem" as required in multiprocessor computer systems. Completelyconnected systems require that each central processing board beconnected with each memory board, resulting in a large number ofconnections and increased distances between the central processingboards and memory boards. As a result, the delays caused by signalstravelling between the boards have limited the speed of multiprocessorsupercomputer systems of the type manufactured by Cray Research, Inc.

In order to overcome the problems of the prior art, the presentinvention employs a novel system of orthogonal connectors to provide alarge number of short connections between circuit boards. The connectorsemploy shape memory metals which are disclosed for use in electronicconnectors in U.S. Pat. No. 4,621,882 to Krumme, issued on Nov. 11,1989, which is incorporated herein by reference. That referencediscloses semi-circular connectors using shape memory metal primarily toprovide zero-insertion-force connectors and include traces on flexiblecircuits to make connection to traces on boards. That reference doesnot, however, refer to the use of such connectors in orthogonalconnectors, nor does it refer to the use of such connectors in closeconfigured completely connected multiprocessor systems as describedherein.

SUMMARY OF THE INVENTION

The present invention relates primarily to the organization andphysical, connection between a number of central processing (CP) boardswith a number of common memory module (CMM) boards in a completelyconnected multiprocessor computer system. More particularly, the presentinvention describes a plurality of stacked CP boards connected onopposing edges to CMM boards which are located in planes perpendicularto those of the CP boards. That orthogonal relationship between the CPboards and the CMM boards allows for a physically small package eventhough the system is completely connected by a sufficient number ofinterconnections to allow each CP board to adequately access the memorycontained in each CMM board.

To accomplish the orthogonal connections between boards, the presentinvention further comprises an orthogonal interconnect module (OIM)which is particularly well suited to connecting boards located inperpendicular planes with a large number of interconnections having ashort length. The distance between the boards is critical to insure thatthe time needed for signal travel does not inordinately limit theprocessing speed of the system.

In addition, the orthogonal interconnect modules also include memorymetal connection devices which allow zero insertion force (ZIF)connections between the OIM and boards. Such a zero insertion forceconnectors are advantageous in that boards may be removed as requiredfor repairs or servicing without damaging them. In the preferredembodiment, the orthogonal interconnect modules with their memory metalconnections can be controlled by heaters to allow the connections toopen for removal or insertion of a board, but when cooled, provide anadequate force to make electrical connection between the board andconnector.

In the preferred embodiment, the orthogonal interconnect modules furtherinclude two flex cables which together supply 520 separate tracesterminating in contact bumps to connect the boards at each end of theOIM. The maximum trace length of the preferred flex cable is less thanfour inches, thus providing a closely spaced short path interconnectbetween the CP boards and the CMM boards of the preferred embodiment. Inaddition, individual cables can be replaced if damaged or otherwiseinoperative.

The resulting preferred package of stacked CP boards surrounded on twosides by CMM boards stacked in perpendicular planes is submerged in athermally conductive, electrically insulated bath to provide sufficientcooling of the boards during operation. In the preferred embodiment, theelectrical power supplies for the system are located directly below theboards, resulting in a compact, high speed multiprocessor basedsupercomputer.

Also in the preferred embodiment, each CMM board include plurality ofmemory chip stacks with each pair of stacks supported by a pair ofcommon memory stack edge boards which, in turn, plug into memory stackconnectors (MSC). Each preferred MSC includes a memory metal portionwhich allows the connector to open and close as desired to provide azero insertion force (ZIF) connection between the edge boards and theCMM board, thus allowing for simplified off-board repair of the memorystacks.

Each MSC preferably includes a flex connector attached to the MSC havinga plurality of trace lines that connects the edge boards to the CMMboard to provide electrical communication lines between the stackedmemory chips and the CMM board. The edge boards have a plurality oftraces connected to the memory chips matching the pattern of those onthe MSC flex connector. The edge boards also contain through-holes toconnect with leads protruding from the memory chips. The chip leads arepreferably bent to provide mechanical and thermal stability to thestacks.

In the preferred embodiment, the CP and CMM boards consist ofmulti-layer circuit boards whose construction is well-known to thoseskilled in the art. The preferred embodiment of the CP boards includes10 locations (five on each of two opposing major sides) forinterconnection with the orthogonal interconnect modules which are, inturn, each connected to a CMM board or other electrical component. Alsoin the preferred embodiment, the CP boards contain areas on the twoedges not connected to CMM's for connection of power to the CP board andthe logic chip packages contained thereon. The preferred CMM boardsinclude sixteen locations (eight on each major side) for interconnectionwith the orthogonal interconnect modules, which are connected to the CPboards.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a CP board stack, surrounding CMM boardsand power supply cabinet;

FIG. 2 is a perspective view of the stacked CP boards surrounded on twosides by orthogonally connected CMM boards;

FIG. 3 is a top view of the CP board stack surrounded on two sides bythe orthogonally connected CMM boards;

FIG. 4 is a side view of the CP board stack and orthogonally connectedCMM boards;

FIG. 5 is a perspective view of the array of orthogonal interconnectmodules shown as connected to one of the CMM boards units;

FIG. 6 is an assembly view of one column of orthogonal interconnectmodules and associated T-beam and heater power supply structure;

FIG. 7 is an exploded assembly view of an orthogonal interconnect modulewithout flex cables;

FIG. 8 is a perspective view of four orthogonal interconnect modules inthe process of attachment to common memory module boards and centralprocessing boards;

FIG. 9 is a cross-section of one end of an orthogonal interconnectmodule;

FIG. 10 is a cross-section of the rear central processing boardalignment showing the lock-out mechanism in the locked out position;

FIG. 11 is a top view of the flexible circuit used in the orthogonalinterconnect module of FIGS. 7 and 8;

FIG. 12 is a top view of the interconnect pattern on each end of theflexible circuit of FIG. 11;

FIG. 13 is a top view of the CP board showing the alignment rails,interconnection pads, power pads, and integrated circuit chipassemblies;

FIG. 14 is a side view of a CP board facing an edge not connected to anorthogonal interconnect module;

FIG. 15 is an enlarged view of a portion of an orthogonal interconnectpad on either a CP or CMM board;

FIG. 16 is a top view of a CMM board;

FIG. 17 is a partial end view of two CMM boards showing the memorystacks and associated memory stack connectors;

FIG. 18 is a close up view of a memory stack showing the memory chipedge boards, and memory stack connectors;

FIG. 19 is a side view of a memory stack edge board including the platedholes in the edge board as well as the trace pattern used forinterconnection of the edge board with the memory stack flex connectortrace pattern;

FIG. 20 is a cross section of an individual memory chip lead as attachedto a CMM stack edge board;

FIG. 21 is a cross section of a memory stack connector showing the tracepattern attached thereto;

FIG. 22 is a top view of the trace pattern of the memory stack flexconnector; and

FIG. 23 is an enlarged view of the memory stack flex connector traces atthe CMM board window.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment of the present invention relates to the highdensity packaging of processors and memory in a multiprocessor system inwhich a plurality of CP circuit boards are connected to a plurality ofCMM circuit boards using orthogonal interconnect modules (OIM) to form acompletely connected system in which each CP board is connected all CMMboards. The application of this technology is designed for speedimprovements, improved heat dissipation, improved packaging density andshortened distances between the CP boards and the full array of CMMboards required for modern multiprocessing supercomputers of the typemanufactured by Cray Research, Inc., assignee of the present invention.

By stacking the CP boards in a closely spaced relationship and arrayingCMM boards along two sides of the CP board stack in planes perpendicularto the planes occupied by the CP boards, each CP board can be connectedto each CMM board, thereby allowing each CP board access to all CMMboards as required to speed the processing of the system. Previousattempts to completely connect such systems has met with limited successas either the number of connections between the CP and CMM boards hasbeen limited or the connection lengths have been prohibitively long,thereby slowing the computing speed of the unit.

By using the board stacking arrangements, orthogonal interconnectmodules (OIM), and densely packed memory boards disclosed in thisapplication, the problems associated with completely connected systemshave been avoided. A more detailed description of the entire systemfollows below.

System Packaging

As shown in FIGS. 1-4, the CP board stack 11 is surrounded on two sidesby CMM board stacks 13 which are aligned in planes perpendicular to theplanes of the CP board stack 11. In the preferred embodiment, the CPboard stack 11 contains eight CP boards 12 and each CMM board stack 13contains four memory boards 14 and space for one additional board 16(see FIG. 3) which can supply a number of required functions such asinterprocessor communications, Input/Output (I/O) translators, clockdistribution and other required system components.

The CP and CMM board stacks 11,13 are preferably immersed in a coolingfluid (not shown) such as FLUORINERT® or a comparable cooling fluid forelectrical components. The fluid is preferably circulated such that itcomes up through the area below the boards into a manifold 6 whichdirects it through the CP board stack 11 from the rear to the front andalso in an upward direction through the CMM board stacks 13. The fluidused to cool the boards then flows out the top of the inner wall 7 ofthe board stack container 4 and is returned down between the inner 7 andouter walls 8 of the board stack container 4 and to the base 2 where itcools the power supplies (not shown) for the system. Once through thepower supplies, the fluid is returned to a reservoir and associatedchiller (not shown). An example of such a cooling system is described inU.S. Pat. No. 4,590,538 to Cray, Jr. issued May 20, 1986, titled"Immersion Cooled High Density Electronic Assembly", assigned to theassignee of the present invention and which is hereby incorporated byreference.

Orthogonal Interconnect Module Array

An overall system view of the preferred embodiment of the orthogonalinterconnect module (OIM) array is shown in FIG. 5 where each of theeight CMM boards 14 (four per side) are connected to each of the eightstacked CP boards 12 by an OIM 20.

Each column of OIM's 20 is located on a T-shaped channel 22 which isused to precisely locate each OIM 20 and prevent it from moving in thesystem. In the preferred embodiment, the OIM's 20 are located on centersapproximately 1.2" apart along the T-shaped channel 22.

In addition to the T-shaped channel 22, each OIM column is also attachedto an electronic heater connection strip 24 as shown in FIG. 6. Thisstrip 24 connects each orthogonal interconnect module's heaters 46 (seeFIG. 6) to power supplies (not shown) to allow the OIM's 20 to be openedas required. As will be understood by referring to the array of FIG. 5,groups of OIM's 20 can be opened in either vertical columns to remove aCMM board 14 or in horizontal rows to remove a CP board 12. Power isdistributed to the heating elements 46 of the OIM's 20 by a power board18 located on top of the CP board stack 11.

Orthogonal Interconnect Module

Referring to FIGS. 7-9, the preferred embodiment of the orthogonalinterconnect module (OIM) 20 consists of a connector body with openings40 disposed on either end but lying in orthogonal planes. The preferredOIM has a cross-section approximately 1.1" square with an overall lengthof approximately 2.4". The contact pressure pads 50 on both ends of thepreferred OIM are located approximately 2.05" apart, center to center.

In the preferred embodiment, the body of the OIM is formed of a cap 32and head piece 30, both preferably of non-glass filledpolyetheretherketone (PEEK), which are formed to nest together. In analternate preferred embodiment, the cap 32 and head piece 30 could beformed of a liquid crystal polymer (LCP). A T-shaped slot 33 to receivethe T-shaped channel 22 is formed at the junction of the cap 30 and headpiece 32. That slot 33 allows the modules 20 to be located on theT-shaped rails 22 within the computer assembly 10.

The head piece 30 preferably contains two bores 38 formed in it, each ofwhich contain a coil spring 36 and ball bearing 34. When the cap 32 andhead piece 30 are assembled, the balls 34 and springs 36 are kept withinthe head piece 30 by the cap 32, with the balls 34 partially protrudinginto the T-shaped slot 33. The balls 34 are sized to cooperate withbores 23 formed in the T-shaped channel 22 to accurately locate themodules 20 along the channel 22 at predetermined locations defined bythe bores 23. Alternately, the connector body could be formed of asingle piece of metal or other material and located by means other thanballs and springs, variations of which will be recognized by thoseskilled in the art.

Each opening 40 on the ends of the preferred embodiment OIM 20 containsthe means required to grasp either a CP 12 or CMM board 14. In thepreferred embodiment, the springs 42,48; contact pressure pads 50; andOIM flex cable contact bumps 66 (see FIGS. 11 and 12) cooperate toresult in a normal force of a minimum of 60 grams between each flexcable contact bump 66 and associated connection pad on either the CP 12or CMM board 14. When open, the distance between the opposing flex cablecontact bumps 66 of the preferred OIM 20 is a minimum of 0.02" widerthan the maximum assembled CP 12 or CMM board 14 thickness. As the basicconstruction of either end of the OIM 20 is similar, only one of theends will be described in detail below.

The cross section of FIG. 9 shows heater leads 47 located in a channel49 formed within the module body opposite the cross-section line. Theheater leads 47 connect the heaters 46 located in the opening with anelectrical power source (not shown). In the preferred embodiment of theOIM 20, each opening 40 contains two heaters 46 although only one heateris used in normal operation, with the second heater operating as abackup to protect against failure of the first heating element. Thepreferred heaters 46 are of a resistance type operating on principleswell-known to those in the industry and, therefore, will not bedescribed further. The actuation cycle required to activate the memorymetal spring 48 to open the OIM 20 is 2.5 amps for 15 seconds in anambient temperature of 25 degrees Centigrade. The preferred heater 46requires 10 watts per inch of length and reaches a temperature of 95degrees Centigrade.

In the preferred embodiment, a connector spring 42 is nested in thecircular opening of the module 20. The connector spring 42 is preferablyformed primarily of a beryllium copper alloy which is chosen for itsspring characteristics. Two ends of the connector spring have a roundededge bar 44 soldered or otherwise attached to the connector spring 42.The rounded edge bars 44 are formed of brass in the preferredembodiment.

In the preferred embodiment, the contact pressure pads 50 have grooves45 in which the rounded edge bars 44 of the connector spring 42 ride.Each contact pressure pad 50 is preferably formed of stainless steel andthe groove 45 is preferably finished to prevent the bars 44 from bindingin the groove 45. In the preferred embodiment, the grooves are peened toprevent binding. Also as shown in FIG. 9, the grooves 45 are alsochamfered 57 to provide uniform pressure across the contact pressure pad50 face 52. The chamfers 57 are preferably formed at a 45° angle to theface 52. In the preferred embodiment, the faces 52 of the contactpressure pads 50 are polished to a grade 16 finish to providesubstantially uniform pressure over their faces 52.

Nested within the connector spring 42 of the preferred embodiment areheaters 46 and a memory metal spring 48 used to force the connectorspring 42 open, thereby moving the contact pressure pads 50 apart. Whenheated by the heaters 46, the memory metal spring 48 attempts tostraighten or flatten itself out which tends to open the connectorspring 42.

The memory metal spring 48 used in the OIM's 20 of the preferredembodiment is given a flattened shape in the austenitic phase above theforming temperature. The memory metal spring 48 is formed into a curledshape while in the martensitic phase and is maintained in themartensitic phase below the austenitic transformation temperature whenthe OIM 20 is closed on a circuit board. In use, the memory metal spring48 is curled and reforms to its flattened shape when heated to itsaustenitic phase. After use, as the spring 48 cools into its martensiticregion, it curls into its desired semi-circular shape. The flattenedshape can again be recovered when the memory metal of the spring 48 isheated into its austenitic region.

When the OIM 20 of the preferred embodiment is to be opened to remove acircuit board, the heaters 46 located next to the memory metal spring 48are activated to warm the spring above the transition temperature sothat the alloy of the spring 48 enters the austenitic phase. In theaustenitic phase, the memory metal spring 48 attempts to reestablish itsflattened shape which opens the connector spring 42, moving the contactpressure pads 50 of the OIM 20 away from each other, thereby allowing acircuit board to be removed without damage.

After a circuit board is replaced, the heater 46 is turned off, allowingthe memory metal spring 48 to cool back to its martensitic phase. Whilecooling, the memory metal spring 48 returns to its curled shape whichallows the connector spring 42 to move the contact pressure pads 50 andassociated OIM flex cable contacts 66 together and into connection withassociated contacts on a circuit board.

To those skilled in the art, the unusual behavior of the memory metalspring 48 is called the shape-memory effect. Shape-memory behavior isconnected with thermoelastic austenitic transformation. The preferredembodiment of the present invention uses the shape memory effect of thememory metal alloys. The use of memory metal in electronic connectors isalso disclosed in U.S. Pat. No. 4,621,882 to Krumme, issued on Nov. 11,1989, which is incorporated herein by reference.

The memory metal used for the spring 48 in the preferred embodiment ofthe present invention is a nickel-titanium alloy which is sometimesreferred to as Nitinol. Although in the preferred embodiment of thepresent invention, Nitinol alloys are described as the best mode ofpracticing the invention, those skilled in the art will also readilyrecognize that a wide variety of shape memory metals havingsuper-elastic qualities could be substituted therefor. Shape memorybehavior is found in a variety of alloys such as Ni-Ti, Ag-Zn, Au-Cd,Au-Cu-Zn, Cu-Al, Cu-Al-Ni, Cu-Au-Zn, Cu-Sn, Cu-Zn, Cu-Zn-Al, Cu-Zn-Ga,Cu-Zn-Si, Cu-Zn-Sn, Fe-Pt, In-Tl, Ni-Al, Ni-Ti, Ni-Ti-X (where X is aternary element), Ti-Co-Ni, Ti-Cu-Ni and others. In the preferredembodiment, nickel-titanium (Nitinol) alloys are used with the specificcomposition of the alloys selected so that the transition temperatureremains well below the ambient operating temperature of the electronicassembly.

To contain the connector spring 42, memory metal spring 48, heaters 46and contact pressure pads 50 within the opening 40 of the OIM 20, theassembly includes keeper plates 53 attached to each side of the opening.The plates 53, preferably stainless steel, are shaped to contain thesprings 42,48 and contact pressure pads 50 in the proper relationship tothe opening 40 of the OIM 20.

Although a memory metal spring 48 and heater 46 assembly is shown in thepreferred embodiment of the OIM 20, it will be appreciated by thoseskilled in the art that a number of other variations could be used toopen and close the contact pressure pads 50, including mechanical orelectromechanical means.

OIM Flex Cable

To make the electrical connections between the boards connected by theOIM 20, its preferred embodiment includes two flex cables 60 as shown inFIG. 8. The flex cables 60 are adapted to be routed along opposite sidesof the connector 20. The OIM flex cable 60 as shown in FIGS. 11 and 12is provided with registration marks 61 in order to align the contactbumps 66 of the OIM flex cable 60 on the contact pressure pad faces 52so that accurate connections can be made with the correspondingconnection pads located on either the CP 12 or CMM boards 14. After thecable 60 is attached to the contact pressure pad faces 52, theregistration marks are trimmed away along with any additional excessmaterial from the flex cables 60.

When in location, the flex cable interconnect areas 64 are located onthe faces 52 of the OIM contact pressure pads 50. In the preferredembodiment, the flex cables 60 are attached to the faces 52 of thecontact pressure pads 50 with an epoxy-based adhesive (not shown). Theflexibility of the cable 60 allows the contact pressure pads 50 to beopened and closed as required without adversely affecting the electricalintegrity of the cable 60.

The OIM flex cable 60 of the preferred embodiment is shown in FIG. 11before attachment to the OIM 20. The flex cable 60 is preferablyapproximately 1.1" wide and approximately 3.3" long (as measured in astraight line from end to end). The trace lines 62 of the flex cable 60follow the substantially rounded curves of the flex cable 60 to preventany unwanted electrical problems due to sharp bends in the flex cable60. The minimum trace length from one end of the preferred flex cable 60to the other end is preferably approximately 3.65 inches with themaximum trace length being approximately 3.99 inches.

The OIM flex cable 60 preferably contains 260 trace lines across its topsurface and has a bottom surface opposite the top trace layers which isessentially a continuous ground layer (not shown). The traces 62 areformed by standard photolithography methods, using either positive ornegative photoresist and masks, in conjunction with electrolytic metalcoating to form the traces 62, ground layer and connection bumps 66 onthe flex cable 60.

In the preferred embodiment, the flex cable base is a flexible organicmaterial (tradename: KAPTON®) having a thickness of 0.001 inches. Thetraces 62 are formed of copper approximately 0.0005 inches thick with amaximum DC resistance of 1.0 ohms per inch. The traces 62 have a widthof approximately 0.002 inches on centers spaced 0.004 inches apart. Theground layer is also preferably formed of copper approximately 0.0005inches thick. The cables 60 are also preferably coated on both sideswith a polyamide layer and associated adhesive which is no more than0.001 inches thick per side, the purpose of which is to protect both thetraces 62 and ground layer.

Each end of the OIM flex cable 60 contains a pattern 64 of contact bumps66, each of which are electrically connected to the end of a trace line62. An enlarged view of the contact bump pattern 64 is shown on FIG. 12.The contact bump pattern 64 as shown, is the same on each end of the OIMflex cable 60.

The preferred contact bump pattern 64 comprises an area approximately1"×0.180" with the contacts 66 forming twenty-six columns and ten rows,resulting in 260 contacts 66 at each end of the flex cable 60. Thecolumns are spaced on 0.040" centers and the rows are spaced on 0.020"centers. The contact bumps 66 of the preferred embodiment areapproximately 0.0060" wide×0.010" long and extend 0.0015-0.0035 inchesabove each trace 62.

In the preferred embodiment, the contact bumps 66 are preferablyelectrolytically deposited on the ends of the flex cable traces 62 inconjunction with standard photolithography techniques using eitherpositive or negative photoresist and the appropriate masks. Thepreferred bump 66 has layers including a copper base layer, copper tracelayer, copper bump, nickel plate and a gold plated top layer. Thecontact bumps 66 are preferably planar to within ±0.0002 inches withinany one of the thirteen individual contact matrices 68 (each matrixcontaining two columns of ten contacts 66) as shown in FIG. 12.

In the preferred system, the traces 62 are used to send electronicsignals in one direction with the copper ground layer (not shown)located opposite the trace side operating as the ground for theelectronic signals. An alternate embodiment of the OIM flex cable 60 canbe constructed without the ground layer opposite the trace side, forsystems in which pairs of traces 62 and associated contact bumps 66 areused to send and receive signals through the flex cable 60. EmitterCoupled Logic (ECL) and other similar uses requiring such paired signalswill be known to those skilled in the art and will not be describedfurther herein.

Although the traces 62, ground plane and contact bumps 66 of thepreferred embodiment are formed of copper, nickel and gold, thoseskilled in the art will understand that other conductive materials couldbe used as well. Additionally, although the metals are electrolyticallydeposited on the flex cable 60, it will be readily understood by thosein the art that metals or other conductive materials could be formed onthe flex cable 60 by sputtering or a variety of other methods.Alternately, the electrically conductive patterns could be etched from asingle layer of material rather than deposited on the flex cable 60 inlayers.

OIM Alignment Mechanism

The contact pressure pads 50 of the preferred OIM 20 also have alignmentfeet 54,56. The feet 54,56 are constructed to cooperate with slots 72,76in alignment rails 70,74 on the CP 12 and CMM boards 14 to accuratelyalign the dense contact bump patterns 64 on the flex cables 60 with thecorresponding pads 82,90 on the CP 12 or CMM boards 14.

The alignment feet 54,56 located on the contact pressure pads 50 of theOIM 20, are of two basic designs. The alignment feet 56 required on thecontact pressure pads 50 which cooperate with the preferred CP boardrails 70 are adapted to allow the CP boards 12 to slide across a row ofOIM's 20, as shown in FIG. 8 and will be described first.

In order to prevent the CP board rails 70,73 from aligning with thealignment feet 56 on the first OIM 20 in a given row of OIM's 20, therails 70,73 include a lock out mechanism as shown in FIG. 10, which isshown in a position to allow the feet 56 to cooperate with the slots 72.The lock out mechanism comprises a mechanical assembly having a rod 71formed with slots 75 which allow the feet 56 to cooperate with the slots72 in the CP board alignment rail 70. The rod 71 can also be rotatedwithin the rail 70 such that the slots 75 are facing towards the CPboard 12 to prevent the alignment feet 56 from cooperating with the railslots 72 because the slots would be filled by the rod 71 in thatconfiguration. After the CP board 12 is in position across the row ofOIM's 20, the rod 71 is rotated to expose the slots 75, allowing thealignment feet 56 to cooperate with the corresponding slots 72 in the CPboard alignment rails 70,73, thus assuring proper location of the CPboard pads 82 and flex cable contact bump patterns 64. The CP boardalignment rail 70 located the farthest in on the CP board 12 is shown inFIG. 10 and, in addition to the lockout mechanism contained by bothrails 70,73 includes a backstop 77 to prevent the CP board 12 from beingpushed too far into the OIM 20.

The alignment rails 74 of the CMM boards 14 do not, however, require alock-out mechanism like that required for the CP board alignment rails70,73 because the CMM boards 14 are not slid across the contact pressurepads 50 of the OIM 20. The CMM board alignment feet 54 are insteaddesigned to allow the CMM boards 14 to be slid into the opening 40 andcontact pressure pads 50 along a longitudinal axis running through thelength of the OIM 20, as shown in FIG. 8. As such, the preferred CMMrails 74 provide slots into which each OIM 20 slides in order to alignthe flex cable bump patterns 64 with the contact pads 90 of the CMMboard 14.

It will be appreciated by those skilled in the art that a large varietyof mechanisms could be used to align the OIM's 20 with either the CPboards 12 or the CMM boards 14 and the details described above for thepreferred embodiment should not be construed to limit the inventionbeyond the claims.

Central Processing Boards

In the preferred embodiment of the present invention, eight centralprocessing (CP) boards 12 are stacked as shown in FIGS. 1-4. The CPboards 12 are preferably of a multi-layer circuit board constructionwellknown to those skilled in the art. In the preferred embodiment, theCP boards 12 have outside dimensions of approximately 6.5"×5.96" withthe CMM boards 14 connected along the 6.5" sides.

Referring to FIGS. 13 and 14, each CP board 12 in the preferredembodiment contains a total of twenty OIM connection pads 82 for OIMconnection of the CP boards 12 to the CMM boards 14. The pads 82 aredistributed such that five OIM connection pads 82 are located on each oftwo opposing edges on each of the two major surfaces of the CP boards.Surrounding the connection pads 82 on two sides are the pairs of CPboard alignment rails 70,73, including lock-out mechanisms (describedabove). On the remaining two opposing edges of the CP boards 12 are pads84 on both major sides (top and bottom) for connecting power to the CPboards 12. These pads 84 could, alternately, include connections forclock distribution, Input/Output (I/O) translators, interprocessorcommunications or a variety of other system components.

The OIM connection pads 82 are located on 1.2" centers along the edgesof the CP board 12. As shown in FIG. 15, each pad 82 contains 260contact pads 83 in an array having twenty-six columns of ten rows ofcontact pads 83 for connection to the contact bumps 66 of the flexcables 60 of the OIM's 20. Each column of pads 83 is located on 0.040"centers and each row is spaced on 0.020" centers. Each pad 83 isapproximately 0.0150" wide×0.0350" long, resulting in a total OIMconnection pad 82 area measuring approximately 1.0×0.18". The OIMconnection pad 82 is placed in relation to the CP board alignment rails70,73 such that when the CP board alignment feet 56 of the OIM 20 are inalignment with the rail slots 72, the contact bumps 66 of each flexcable 60 align with the corresponding contact pads 83 of the OIMconnection pad 82.

In the preferred embodiment, the CP board contact pads 83 are preferablyelectrolytically deposited on the CP boards 12 in conjunction withstandard photolithography techniques using either positive or negativephotoresist and the appropriate masks. The preferred pad 83 includes acopper base layer, nickel plate layer and a gold plated top layer.

Although the contact pads 83 of the preferred embodiment are formed ofcopper, nickel and gold, those skilled in the art will understand thatother conductive materials could be used as well. Additionally, althoughthe metals are electrolytically deposited on the board, it will bereadily understood by those in the art that metals or other conductivematerials could be formed on the CP boards by sputtering or a variety ofother methods. Alternately, the electrically conductive patterns couldbe etched from a single layer of material rather than deposited on theCP board 12 in layers.

Common Memory Module Board Assembly

A top view of a common memory module (CMM) board 14 is shown in FIG. 16.The preferred embodiment of the CMM board 14 is of a multi-layer circuitboard construction well known to those skilled art. The preferred board14 is 10"×10" and contains an 8×4 array of memory modules 99, eachmodule 99 having two individual memory chip stacks 92 electricallyconnected to the OIM connection pads 90. Also located on the board are avariety of logical circuits 93, as well as other associated systemcomponents as required for operation of the CMM board.

All of the memory modules 99 with chip stacks 92 are located on the samemajor side of the CMM board 14 as shown in FIG. 16. The logic circuits94 used to control the flow of information into and out of each memorychip stack 92 are located on the opposite side of the boards 14 from thememory modules 99. Referring to FIG. 17, when the CMM boards 14 arelocated in the constructed computer assembly 10, the distance betweenthe top of the edge boards 96 on one CMM board 14 to the bottom, logicside of the adjacent CMM board 14 is preferably approximately 0.0726inches (nominal).

Referring to FIG. 16, one edge of the preferred CMM board 14 contains atotal of sixteen OIM connection pads 90, eight on each major side of theboard 14, and the CMM board alignment rails 74 required to connect withthe OIM 20 to the pads 90. On the opposite edge of the CMM board 14 areconnection pads 91 for power and other functions such as clockdistribution, Input/Output (I/O) translators and other system componentsconnected to the CMM board 14.

The OIM connection pads 90 of the CMM board 14, like those of the CPboard, are located on 1.2" centers along the edge of the board 14. Eachpad 90, shown enlarged in FIG. 15, contains 260 contact pads 83 in anarray having twenty-six columns of ten rows of contact pads 83. Eachcolumn is located on 0.040" centers and each row is spaced on 0.020"centers. Each pad 83 is approximately 0.0150" wide×0.0350" long,resulting in a total OIM connection pad 90 area measuring approximately1.0"×0.18". The OIM connection pad 90 is placed in relation to the CMMboard alignment rails 74 such that when the CMM board alignment feet 54of the OIM 20 are in alignment with the rail slots 76, the 260 contactbumps 66 of each flex cable 60 align with the corresponding 260 contactpads 83 of the CMM board OIM connection pad 90.

In the preferred embodiment, the contact pads 90 are preferablyelectrolytically deposited on the CMM boards 14 in conjunction withstandard photolithography techniques using either positive or negativephotoresist and the appropriate masks. The preferred pad 83 includes acopper base layer, nickel plate and a gold plated top layer.

Although the contact pads 83 of the preferred embodiment are formed ofcopper, nickel and gold, those skilled in the art will understand thatother conductive materials could be used as well. Additionally, althoughthe metals are electrolytically deposited on the board, it will bereadily understood by those in the art that metals or other conductivematerials could be formed on the CMM boards by sputtering or a varietyof other methods. Alternately, the electric conductive patterns could beetched from a single layer of material rather than deposited on the CMMboard 14 in layers of.

Memory Stack Detail

As shown in FIGS. 16 through 18 in the preferred embodiment each memorymodule 99 is comprised of two individual stacks 92 of twenty memorychips 97 connected to two edge boards 96. The edge boards 96 areconnected to memory stack connectors (MSC) 95 located on the CMM boards14. The memory chips 97 are connected on two sides to the edge boards96.

FIG. 19 shows a side view of a preferred single edge board 96 withinterconnection holes 100 and a trace pattern 102 located on the side ofthe board which slides into the MSC 95. The preferred edge board 96 is2.24" long, 1.08" high and 0.03" thick. The edge board 36 includesregistration holes 104 for aligning the memory stack 92 in the MSC 95.In the preferred embodiment the edge board holes 100 are located in tworectangular arrays 106,108 as shown in FIG. 19, each array 106,108 forconnection to an individual stack of memory chips 97 and measuringapproximately 0.54" long and 0.72" high. The holes 100 are located innineteen columns and twenty rows. The columns are located on 0.03"centers and the rows are located on 0.04" centers. As shown in FIG. 20,each hole 100 is 0.016" in diameter and is drilled through a 0.02"diameter contact pad 103 formed on the board 96.

The preferred edge board trace pattern 102 is approximately 1.876" longand 0.09" high and is located approximately 0.05" from the bottom edge101 of the board 96. The pattern of the traces 102 is shown havingvarying widths which are dependent on whether the traces are designed tocarry signals or are for power/ground purposes. The edge boards 96 areconstructed using standard printed circuit board technology to connectthe holes 100 with the trace pattern 102. Such details are well-known tothose skilled in the art and will not be discussed further herein.

The memory chips 97 are connected to the edge boards 96 by thirty-eighttab tape leads 98, nineteen on each of two sides of the chip 97, eachlead 98 being connected to contact bumps 107 on the chips 97 as shown inFIG. 20. Such tab tape lead-bump connections are well-known to thoseskilled in the art and will not be further described herein.

The tab tape leads 98 of the preferred embodiment have a length of0.1130" with an oblique section 105 formed therein. The preferred angleof the oblique section 105 is approximately 22 degrees from thevertical. The leads 98 are formed with oblique sections 105 to providemechanical stability to the memory stacks 92 as well as to provide formismatched thermal expansion between the various components. The leads98 are connected to the edge board holes 100 using flow-through soldertechnology, which is well-known to those skilled in the art and will notbe further described herein.

The modular construction of the memory modules 99 allows them to beremoved and/or replaced on the CMM board, thus simplifying repair anddecreasing downtime of the system by allowing modules 99 to be swappedfor offboard repair.

Memory Stack Connector

In the preferred embodiment of the present invention, the memory stackconnectors (MSC) 95 of the CMM boards 14 are used to provide a zeroinsertion force connector to connect the memory stack edge boards 96 ofa memory module 99 to the CMM board 14. Each CMM board 14 preferablycontains sixty-four separate MSC's 95, with two MSC's 95 required toconnect each memory module 99 to the CMM board 14, as shown in FIGS.16-18.

The preferred MSC 95 is biased in the closed position, meaning thatpoints 120 and 122, as shown in FIG. 21, on the MSC 95 are at theirclosest position to one another. Also in the preferred embodiment, eachMSC 95 can be divided into two halves with one portion, containing point122 and shown on the left in FIG. 21, being inactive and the oppositeportion containing point 120 being active. The active-inactivedistinction is based on the concept that during opening and closing ofthe MSC 95 only the active portion moves, as further described below.

The preferred MSC 95 includes a monolithic spring section 110,preferably of a beryllium copper alloy, to provide a constant springforce attempting to bias the MSC 95 in the closed position. When closedon an edge board trace pattern 102, each MSC 95 will preferably providea normal contact force of between 60 and 125 grams per trace. Thoseskilled in the art will recognize that a variety of metals and othermaterials having sufficient spring characteristics could be substitutedfor the beryllium copper alloy of the preferred embodiment.

The preferred MSC 95 also includes a shape memory metal portion 112,heater 114 to activate the memory metal portion, MSC flex connector 166and additional insulation 118 between the bottom of the MSC 95 and CMMboard 14 to protect the board 14 from heat damage when the heater 114 isused, all as shown in the cross section of FIG. 21. The preferred heater114 is of a resistance type operating on principles well-known to thosein the industry and, therefore, will not be described further. Theheater actuation cycle required to activate the memory metal portion 112to open the MSC 95 is 2.5 amps for 15 seconds with an ambienttemperature of 25 degrees Centigrade. The preferred heater 114 requires10 watts per inch of length and reaches a temperature of 95 degreesCentigrade. When open, the preferred MSC 95 opens a minimum of 0.02"wider between points 120 and 122 than the maximum edge board 96thickness.

The shape memory metal portion 112 of the preferred MSC 95 is used toopen and close the MSC 95 as desired. In the preferred embodiment, theshape memory metal 95 is preferably formed of a nickel titanium alloy,or Nitinol. When heated, the memory metal portion 112 tends to flattenwhich in turn opens the MSC connector 95 to allow either removal orinsertion of a memory module 99.

The memory metal used in the preferred MSC 95 of the present inventionis given a flattened shape in the austenitic phase above the formingtemperature. The memory metal is formed into a curled shape while in themartensitic phase and is maintained in the martensitic phase below theaustenitic transformation temperature when the MSC 95 is closed on anedge board 96. In use, the memory metal attempts to reform to itsflattened shape when heated to its austenitic phase. After use, as thememory metal cools into its martensitic region, it returns to itsdesired curled shape. The flattened shape can again be recovered whenthe memory metal is heated into its austenitic region.

When the MSC 95 of the preferred embodiment is to be opened to removethe edge boards 96 of a memory module 99, the heater 114 located next tothe memory metal 112 is activated to warm the metal above the transitiontemperature so that the alloy enters the austenitic phase. In theaustenitic phase, the memory metal 112 attempts to reestablish itsflattened shape which moves point 120 away from point 122, moving theMSC flex connector 116 away from the edge boards 96, thereby allowingthe memory module 99 to be removed without damage.

After the memory module 99 and its associated edge boards 96 arereplaced, the heater 114 is turned off, allowing the memory metal 112 tocool back to its martensitic phase. While cooling, the memory metal 112returns to its curled shape which allows the monolithic spring section110 to move the MSC flex connector 116 into connection with theassociated trace pattern 102 on the edge board 96.

To those skilled in the art, the unusual behavior of the memory metalportion 112 of the MSC 95 is called the shape-memory effect.Shape-memory behavior is connected with thermoelastic austenitictransformation. The preferred embodiment of the present invention usesthe shape memory effect of the memory metal alloys. The use of memorymetal in electronic connectors is also disclosed in U.S. Pat. No.4,621,882 to Krumme, issued on Nov. 11, 1989, which is incorporatedherein by reference.

The memory metal used in the preferred embodiment of the MSC 95 is anickel-titanium alloy which is sometimes referred to as Nitinol.Although in the preferred embodiment of the present invention, Nitinolalloys are described as the best mode of practicing the invention, thoseskilled in the art will also readily recognize that a wide variety ofshape memory metals having super-elastic qualities could be substitutedtherefor. Shape memory behavior is found in a variety of alloys such asNi-Ti, Ag-Zn, Au-Cd, Au-Cu-Zn, Cu-Al, Cu-Al-Ni, Cu-Au-Zn, Cu-Sn, Cu-Zn,Cu-Zn-Al, Cu-Zn-Ga, Cu-Zn-Si, Cu-Zn-Sn, Fe-Pt, In-Tl, Ni-Al, Ni-Ti,Ni-Ti-X (where X is a ternary element), Ti-Co-Ni, Ti-Cu-Ni and others.In the preferred embodiment, nickel-titanium (Nitinol) alloys are usedwith the specific composition of the alloys selected so that thetransition temperature remains well below the ambient operatingtemperature of the electronic assembly.

MSC Flex Connector

To make electrical connection between the trace pattern 102 on the edgeboards 96 and the CMM board 14, the MSC 95 preferably has a tracepattern located on a flexible connector 116 which is connected tomonolithic spring 110 of the MSC 95 as shown in FIG. 21. Referring alsoto FIG. 22, the trace pattern window 121 on the flex connector 116contacts the trace pattern 102 located on the common memory stack edgeboards 96 at point 120 when the edge boards 96 are inserted into aclosed MSC 95.

In the preferred embodiment, the MSC flex connector 116 is attached tothe MSC 95 with an epoxy-based adhesive (not shown). The flex connector116, as shown in FIG. 22 is preferably approximately 1.93" high and0.84" wide. A detail of the trace pattern of the preferred MSC flexconnector 116 is also shown in FIG. 22. The pattern is exposed in twolocations 118,121. The CMM board contact window 118 is used to connectthe traces 124, 125 to the CMM board 14 and the edge board contactwindow 121 is used to contact the edge board trace pattern 102 when theflex connector 116 is attached to the MSC 95. In an alternate preferredembodiment of the MSC flex connector 116, a power/ground layer (notshown) can also exposed at window 123, which is attached to the inactiveportion of the MSC 95 at point 122 for connection to a correspondingarea on an alternate preferred embodiment of the edge boards 96.

When the flex connector 116 is attached to the MSC 95 in the preferredembodiment, the edge board contact window 121, 0.080" wide across theflex connector 116, is centered 0.120" down from the top of the MSC 95.The CMM contact window 118, preferably 0.025" wide across the flexconnector 116, is approximately 0.065" from the active portion of themonolithic spring 110 of the MSC 95. The flexibility of the connector116 allows the MSC 95 to be opened and closed for a large number ofcycles without adversely affecting the integrity of the traces 124,125contained in the flex connector 116.

The MSC flex connector 116 preferably contains signal trace lines 125,four isolated power/ground planes with three coplanar access traces 124,an edge board contact window 121, and a CMM board contact window 118across its surface. The features are formed by standard photolithographymethods, using either positive or negative photoresist and masks, inconjunction with electrostatic metal coating to form the traces 125 andpower/ground layers 124.

In the preferred embodiment, the MSC flex connector signal 125 andpower/ground metal layers 124 are formed of one ounce copper with athickness of 0.0014". The traces 124,125 are formed with a space of0.009" between them. The signal traces 125 are 0.005" wide and containbreak-away portions 129 as shown in FIG. 23 which neck down to 0.002"for break away construction at the CMM board contact window 118. Thepower/ground traces are 0.019" wide in the flex connector 116 and narrowdown to two portions 127, each 0.005" wide, each of which also containbreak-away portions 128 which are 0.002" wide for break-awayconstruction at the CMM board contact window 118. The bonding of the MSCflex connector traces 126,127 as shown in FIG. 23 to the CMM circuitboard 14 is accomplished by through-hole bonding of the traces 126,127as disclosed in copending U.S. patent application filed on Jun. 28, 1990with Ser. No. 07/545,271, titled "Flexible Automated Bonding Apparatusfor Chip Carriers and Printed Circuit Boards" which is incorporatedherein by reference.

The copper used to form the signal traces 125 and power/ground traces ofthe preferred flex connector 116 is coated with a nickel barrier layer,30 to 50 microinches thick, and an 80 to 120 microinch thick layer ofsoft gold (Knoop hardness of 60 to 90). The nickel layer is added as abarrier to prevent migration of the gold layer into the copper while thegold layer aids in making electrical connections while being lesssubject to corrosion than copper. Between the signal and power/groundlayers, the preferred embodiment of the flex connector 116 includes a0.003" layer of adhesive and a flexible organic material, with atradename of KAPTON®. In addition, a 0.002" layer of adhesive andKAPTON® covers both the signal and power/ground layers except whereexposed at the edge board 121 and CMM board contact windows 118.

Although the signal and power/ground layers of the preferred embodimentof the MSC flex connector 116 are formed of copper, nickel and gold,those skilled in the art will understand that other conductive materialscould be used as well. Additionally, although the metals areelectrolytically deposited on the flex connector 116, it will be readilyunderstood by those in the art that metals or other conductive materialscould be formed on the flex connector 116 by sputtering or a variety ofother methods.

Although a specific embodiment has been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. For example, differenttypes of electrical circuit boards, different electrical connectionpatterns, different metals, such as beryllium copper or memory metalssuch as nickel-titanium, or different barrier metals than thosedisclosed in the detailed description could be used. This application isintended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

We claim:
 1. A circuit board apparatus adapted to store large amounts ofinformation in a densely packed configuration comprising:a circuitboard; a plurality of information storage modules having a plurality ofstacks of integrated circuit devices, each stack having a plurality ofintegrated each of said devices being electrically and mechanicallyconnected to a plurality of edge boards which maintain said devices insaid stacked relationship; retaining means attached to said circuitboard for retaining said edge boards on said circuit board, saidretaining means further comprising a memory metal portion forcing saidretaining means into an open position in which said edge boards can benon-destructively removed from or inserted into said retaining means,said retaining means further being normally biased in a closed positionin which an edge board is retained within said retaining means forconnection to said circuit board; and connection means for electricallyconnecting said edge boards to said circuit board, said connection meansfurther comprising a substantially flexible electrical connectorelectrically connecting said information storage modules and circuitboard; whereby electrical communication between said modules and saidcircuit board can be performed.
 2. The apparatus of claim 1, whereineach of said integrated circuit devices further comprises electricallyconductive leads bonded to said edge boards to retain said integratedcircuit devices in said stacked relationship.
 3. The apparatus of claim2, wherein said leads are flow-through solder bonded in electricallyconductive holes in said edge boards.
 4. The apparatus of claim 1,wherein each of said information storage modules further comprises meansfor enhancing mechanical stability of the modules and compensating forthermal expansion of the integrated circuit devices.
 5. The apparatus ofclaim 4, wherein said means further comprises at least one ben din eachof said leads between the integrated circuit devices and edge boards. 6.The apparatus of claim 1, wherein said memory metal portion is comprisedof an alloy exhibiting martensitic transformation behavior.
 7. Theapparatus of claim 6, wherein said martensitic transformation behavioris temperature-induced.
 8. The device of claim 7, further comprising atleast one heating means for heating said memory metal portion.
 9. Theapparatus of claim 1, wherein said memory metal portion is comprised ofan alloy exhibiting a shape memory behavior.
 10. The apparatus of claim6, wherein said memory metal portion is comprised of an alloy containingat least nickel and titanium.
 11. The apparatus of claim 6, wherein saidmemory metal portion is comprised of an alloy selected form the groupconsisting of Ag-Zn, Au-Cd, Au-Cu-Zn, Cu-Al, Cu-Al-Ni, Cu-Au-Zn, Cu-Sn,Cu-Zn, Cu-Zn-Al, Cu-Zn-Ga, Cu-Zn-Si, Cu-Zn-Sn, Fe-Pt, In-Tl, Ni-Al,Ni-Ti, Ni-Ti-X (where X is a ternary element), Ti-Co-Ni, Ti-Cu-Ni, andmixtures thereof.
 12. The apparatus of claim 1, wherein said connectionmeans further comprises at least one exposed trace pattern on saidflexible electrical connector to allow electrical communication betweensaid information storage modules and said circuit board.
 13. Theapparatus of claim 12, wherein each of said edge boards furthercomprises an exposed trace pattern adapted for electrical contact withsaid flexible electrical connector.
 14. The apparatus of claim 12,wherein said circuit board further comprises a plurality of exposedtrace patterns adapted for electrical contact with said flexibleelectrical connector.